EP1198675A1

EP1198675A1 - System and method for manipulating and controlling fluid flow over a surface

Info

Publication number: EP1198675A1
Application number: EP00948708A
Authority: EP
Inventors: Philip P. Truax; Daniel N. Miller; Jeffrey W. Hamstra; Patrick J. Yagle
Original assignee: Lockheed Corp; Lockheed Martin Corp
Current assignee: Lockheed Martin Corp
Priority date: 1999-07-16
Filing date: 2000-07-14
Publication date: 2002-04-24
Also published as: US6682021B1; WO2001006134A1; US6371414B1; AU6217200A

Abstract

The present invention provides a system and method for actively manipulating and controlling aerodynamic or hydrodynamic fluid flow (10) over a surface. More specifically, the present invention provides a system and method to control aerodynamic or hydrodynamic fluid flow behavior of a ducted fluid flow using very-small-scale effectors (12). The system and method for actively manipulating and controlling fluid flow over a surface (20) includes the placement of arrays (14) of very-small-scale effectors on ducted surfaces bounding the ducted fluid flow. These very-small-scale effectors actively manipulate the boundary layer to control the flow behavior of the ducted fluid flow and suppress or prevent flow separation within the primary fluid flow (10A).

Description

SYSTEM AND METHOD FOR MANIPULATING AND CONTROLLING FLUID FLOW OVER A SURFACE

TECHNICAL FIELD

This invention relates generally to manipulating fluid flow over a surface More specifically, this invention relates to both actively and passively manipulating ducted fluid flow over an aerodynamic or hydrodynamic surface The fluid flow over the ducted surface is manipulated by very-small-scale effectors at the surface, wherein these very-small-scale effectors achieve a desired fluid flow behavior over the surface

RACK G UND ART One of the most commonly used methods to control local boundary layer separation withm ducted systems is the placement of vortex generators upstream of the layer separation within a natural fluid flow Vortex generators are small wmg like sections mounted on the inside surface of the ducted fluid flow and inclined at an angle to the fluid flow to generate a shed vortex The height chosen for the best interaction between the boundary layer and the vortex generator is usually the boundary layer thickness The principle of boundary layer control by vortex generation relies on induced mixing between the primary fluid flow and the secondary fluid flow The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer Fluid particles with high momentum m the stream direction are swept along helical paths toward the duct surface to mix with and, to some extent replace low momentum boundary layer flow This is a continuous process that provides a source to counter the natural growth of the boundary layer creating adverse pressure gradients and low energy secondary flow accumulation

The use of vortex generators in curved ducts to reduce distortion and improve total pressure recovery has been applied routmely Many investigations have been made in which small-geometry surface configurations effect turbulent flow at the boundary layers Particular attention has been paid to the provision of surfaces in which an array of small longitudinal elements extend over the turbulent boundary layer region of a surface m the direction of fluid flow over the surface, to reduce momentum transport or drag Some experimental results mdicate that net surface drag reductions of up to about 7% can be achieved However, these structures, used to induce vortices, are fixed and address vortex generation with a fluid flow It would be desirable to provide a mechanism to actively manipulate the vortex generation withm a dynamic fluid flow condition

As computers increasingly leave fixed locations and are used in direct physical applications, new opportunities are perceived for applymg these powerful computational devices to solve real world problems in real time To exploit these opportunities systems are needed which can sense and act Micro-fabricated Electro- Mechanical Systems (MEMS) are perfectly suited to exploit and solve these real world problems

MEMS offer the integration of micro-machmed mechanical devices and microelectronics Mechanical components in MEMS, like transistors m micro-electronics, have dimensions that are measured m microns These electro-mechanical devices may include discrete effectors and sensors More than anything else, MEMS is a fabrication approach that conveys the advantages of miniaturization, multiple components and microelectronics to the design and construction of mtegrated electro-mechanical systems To utilize an individual MEMS device to control or manipulate microscopic conditions can be useful. However, it would be desirable to achieve macroscopic effects by manipulating microscopic conditions These microscopic effects can be achieved by the MEMS passively or by actively manipulating these devices. Furthermore, in a system utilizing active manipulation, it may be desirable to utilize a sophisticated control system in conjunction with a large array of MEMS devices to control and manipulate such macroscopic effects.

DISCLOSURE OF INVENTION

The present invention provides a system and method for manipulating and controlling aerodynamic or hydrodynamic fluid flow over a surface that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods used of fluid flow control.

More specifically, the present mvention provides a system and method to control aerodynamic or hydrodynamic fluid flow behavior of fluid flow usmg very-small-scale effectors. The system and method for manipulating and controlling fluid flow over a surface includes the placement of arrays of very-small-scale effectors on ducted surfaces bounding the ducted fluid flow In one embodiment, these effectors may passively manipulate the ducted fluid flow

In a second embodiment, these very-small-scale effectors are actively manipulated to control the flow behavior of the fluid flow and prevent natural flow separation withm the pπmary fluid flow.

An additional embodiment of the present invention mcludes the use of MEMS devices as the very-small- scale effectors. In this case the very-small-scale effectors are smaller than a duct boundary layer thickness of a ducted fluid flow Furthermore these very-small-scale effectors may be dimensioned at an order of one-tenth of the boundary layer thickness

A third embodiment of the present invention further senses the flow conditions withm in the primary and secondary fluid flow, with a flow sensor system. This relieves computational burdens imposed by prior art systems which failed to sense actual conditions and relied on computationally intensive mathematical models to determine the fluid flow conditions. Further, this eliminates the inaccuracies associated with such mathematical models and provides real time actual feedback. A control system is employed to actively direct the array of very- small-scale effectors m response to the sensed flow conditions to produce a desired fluid flow withm the ducted fluid flow.

Manipulating boundary layer conditions for a ducted fluid flow expands the possible geometries available for ducting systems used to contain such a fluid flow. This is highly desirable where exotic geometries are required. One example of such use may be in a low-observable tactical aircraft. However, the present invention need not be limited to tactical aircraft as low-observable technology has many applications as known to those skilled in the art.

Manipulating a ducted fluid flow can result in reduced fatigue and cyclical stress effects on downstream components located withm the fluid flow, such as an aircraft engme or turbine, as the adverse pressure gradients withm the duct can be rnimmized or eliminated. Included m the present invention is the possibility of using effectors that are actively controlled either as pulsatmg effectors that are either on or off (non-pulsating) or are actively controlled through a system that uses small sensors (like MEMS) to control the effectors. The operational performance of components, such as an aircraft engme or turbine, located withm the ducted fluid flow can be enhanced by actively monitoring and controlling the fluid flow behavior to prevent operational failures such as engme stalling induced by fluid flow through the engme.

The present invention provides an important technical advantage by allowing a dynamic pπmary fluid flow to be manipulated by an array of very-small-scale effectors to achieve a greater pressure recovery of the pnmary fluid flow

The use of vortex generators m curved ducts to reduce distortion and improve total pressure recovery has been applied routmely Recent investigations have shown that a "global" approach to the application of conventional size vortex generators or vortex generator jets works better than the older approach which was specifically aimed at the prevention of boundary layer flow separation. The present invention applies to the use of very-small-scale devices (effectors) to accomplish this "global" approach to flow control. The present invention provides another technical advantage by preventing or limiting natural flow separation withm a ducted fluid flow This is achieved by manipulating an array of very-small-scale effectors according to conditions sensed withm the fluid flow to achieve a macroscopic effect Individually, these very-small-scale effectors induce vortex generation at the boundary layer. These vortices mduce mixing between the pπmary fluid flow and the secondary fluid flow

The mixmg is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum m the stream direction are swept along helical paths toward the duct surface to mix with and, to some extent replace low momentum boundary layer flow. By manipulating the vortex generation across an array of very-small-scale effectors, a macro-scale influence can be achieved. This provides a macro-scale source to counter the natural growth of the boundary layer, thereby eliminating low energy secondary flow accumulation withm the pπmary fluid flow. In an additional embodiment, smce the induction of these vortices can be actively controlled, it is possible for the present invention to respond to changmg conditions effecting the dynamic fluid flow

Moreover, the present invention provides yet another technical advantage by allowing a greater flexibility m the design of a ducting system to be associated with the primary fluid flow This flexibility allows both inlet and exhaust ductmg to take exotic or serpentine shapes as is often required by other engineering design constraints, such as those imposed m the design of a low-observable tactical aircraft.

Yet another technical advantage provided by the present mvention, when applied to a low-observable tactical aircraft, is the mcreased design flexibility m providing ductmg systems optimized to minimize radar reflections, disperse exhaust signatures, and provide adequate airflow to the components located withm the ducted fluid flow of a low-observable tactical aircraft.

BRIEF DESCRIPTION OF DRAWINGS

A more complete understandmg of the present invention and the advantages thereof may be acquired by referring to the following descπption, taken in conjunction with the accompanymg drawings in which like reference numbers indicate like features and wherem:

FIGURE 1A illustrates conceptually a passive embodiment of the present invention;

FIGURE IB illustrates conceptually an active embodiment of the present mvention; FIGURE 2A depicts a fluidic effector;

FIGURE 2B depicts a pulsmg effector;

FIGURE 2C depicts a synthetic jet effector;

FIGURE 2D depicts a microbubble effector; FIGURE 3A illustrates flow conditions within a serpentine duct without the present invention;

FIGURE 3B illustrates secondary flow characteristics within a serpentine duct not usmg the present invention,

FIGURE 3C is a cross section depicting pressure gradients within a serpentine duct without the use of the present mvention, FIGURE 4A illustrates flow conditions within a serpentine duct with the present mvention;

FIGURE 4B illustrates secondary flow characteπstics withm a serpentine duct using the present invention;

FIGURE 4C is a cross section depicting pressure gradients withm a serpentine duct with the use of the present invention, FIGURE 5 A depicts a MEMS shear sensor,

FIGURE 5B depicts a MEMS pressure sensor;

FIGURE 5C depicts a MEMS velocity sensor,

FIGURE 6 depicts an embodiment of the present invention similar to FIGURE 1, wherem flow sensors are incorporated on the surface over which the flow takes place; FIGURE 7 depicts a low-observable duct;

FIGURE 8A depicts natural flow separation within a serpentine duct; and

FIGURE 8B depicts reduced flow separation when the present invention is employed within a serpentine duct.

MODES FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present mvention are illustrated m the FIGURES, like numerals bemg used to refer to like and corresponding parts of various drawings.

The present invention provides a system and method for manipulating aerodynamic or hydrodynamic fluid flow over a surface that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods used of fluid flow control

More specifically, the present mvention provides a system and method to prevent or minimize the natural boundary layer separation of an aerodynamic or hydrodynamic fluid flow through the use of very-small-scale effectors The system and method for preventing or mmimizmg this boundary layer separation withm a fluid flow over a surface includes the placement of arrays of very-small-scale effectors on ducted surfaces boundmg ducted fluid flow These very-small-scale effectors modify the flow behavior of the ducted fluid flow and prevent flow separation withm the pπmary fluid flow. The present inventions enables new and unproved designs of low-observable tactical aircraft Low- observable m part takes mto consideration such as detection by radar and the radar cross section associated with a low-observable aircraft

One method to detect aircraft mvolves the use of radar However, not all objects or aircraft reflect the same amount of radar waves, as is known by those skilled in the art In a low-observable aircraft one would want to reflect as little radar energy as possible to a radar receiver, enabling the plane to go undetected at closer ranges The amount of radar energy that is reflected by an object can be defined by its radar cross-section To define the radar cross-section of a target, one calculates the size of a sphere which would reflect the same amount of radar energy as the aircraft that was measured The radar cross-section m square meters is then the area of a circle of the same diameter as the imaginary sphere

Radar cross-section is not necessarily defined by aircraft size, but is more closely related to its design and construction Curved surfaces reflect energy in many directions Therefore, curved surfaces have been historically avoided m favor of flat surfaces Flat surfaces, like the facets of a diamond, reflect energy m the limited directions of the designers' choice— namely, away from detecting receivers for a low-observable aircraft As the computational power of computers have mcreased designers need no longer be limited to faceted surfaces, rather surfaces, including curved surfaces, may be modeled and optimized to minimize the amount of radar energy reflected to an detecting receiver

One problem source of radar signatures from aircraft has been associated with the engine ductmg In some mstances this has been used as an identifier of the aircraft Modern radars can look down engine inlets to bounce returns off the highly reflective compressor blades (and m many cases identify them by counting the rotatmg blades)

Radar, however, is not the only method of aircraft detection To reduce their vulnerability to heat- seeking detection systems, low-observable aircraft may use slit-like exhausts Exhaust may emerge in cool, diffuse fans rather than hot, concentrated streams FIGURE 1A illustrates conceptually one embodiment of the present mvention FIGURE 1A describes a fluid flow 10, which may be an air, gas or liquid, flowing over surface 20, wherem surface 20 has an array of very-small-scale effectors 12 Array 14 of very-small-scale effectors 12 allows the flow characteristics of fluid flow 10 to be manipulated as it flows over surface 20 Fluid flow 10 will develop mto primary flow 10A and secondary flows 10B The very-small-scale devices (on the order of one-tenth of the boundary layer thickness) can be fixed

(effectors) like miniature vortex generators or vortex generator jets The miniature vortex generators can be fabπcated in many ways mcludmg MEMS techniques or applied as an applique or cast mto the surface

In a second embodiment, illustrated m FIGURE IB, array 14 of very-small-scale effectors 12 are linked to a control system 18 by a communication pathway 16 Communication pathway 16 may be a hard- wired system, a fiber optic system, a wireless communications system, or the like, as known to those skilled in the art

Control system 18 will also receive mput from a fluid flow sensor system 22 Fluid flow sensor system 22 will provide data on flow characteπstics of fluid flow 10 (mcludmg pπmary fluid flow 10A and second fluid flow 10B) to control system 18 Fluid flow sensor system 22 may detect the flow behavior characteπstics of fluid flow 10 withm the fluid flow, or the fluid flow sensor system 22 may comprise a series of sensors mounted on surface 20

Array 14 is composed of several very-small-scale effectors 12 Very-small-scale effectors 12 will be smaller than the boundary layer thickness of the fluid flow 10 over surface 20 In some instances and embodiments, very-small-scale effectors 12 may be one-tenth the thickness of the boundary layer or smaller For this reason, microfabricated electro-mechanical structures (MEMS) are chosen for these very-small-scale effectors 12

FIGURES 2A-2D illustrate many examples of microfabπcated electro-mechanical structures (MEMS) which may be used as these very-small-scale effectors FIGURE 2A depicts a fluidic effector creatmg secondary flows 10B as primary fluid flow 10 passes over fluidic effector 24 FIGURE 2B depicts a pulsmg effector A fluidic oscillator alternates flow between two outflow legs by mjectmg high pressure on either side of the nozzle oπfice Injecting at Input 1 causes flow to exit the device at Output 2, and injecting at Input 2 causes flow to exit the device at Output 1 The Input flow can come from a like, but smaller device (Second Stage) or from a mechanically dπven valve (First Stage) FIGURE 2D depicts a synthetic jet effector This type of effector uses a vibratmg diaphragm which bounds a cavity to generate an air jet The oscillating surface draws fluid mto the cavity from all directions and then expels it m a narrow jet The resultant pulsed jet has no net mass flow FIGURE 2E presents a micro-bubble effector where micro-bubbles 20 expand based on internal pressure to manipulate secondary flow 10B The effectors listed above are examples of possible MEMS devices which may be used to manipulate pπmary fluid flow 10 These very-small-scale effectors may be placed m arrays on surface 20 and controlled by control system 18 coupled to array 14 via communication pathway 16 Any of these very-small-scale effectors 12 may be used to mduce and manipulate vortex formation to control the natural layer separation withm the fluid flow 10

Surface 20 need not be limited to a free surface In many mstances, surface 20 will be part of a ducted surface, as illustrated in FIGURES 3A and 4A FIGURE 3 A depicts a serpentine duct 25 with no control of the secondary flow behavior 10B of fluid flow 10 withm this duct 25 Lmes 28 mdicate instabilities or pressure holes (low pressure areas) 30 within fluid flow 10 withm duct 24 FIGURE 3B illustrates the natural secondary flow characteπstic 28 withm duct 25

FIGURE 4A agam illustrates a serpentine duct 25 However, this serpentine duct has arrays 14 of very- small-scale effectors 12 withm the ducted surface 20 of duct 25 FIGURE 4B presents the modified secondary flow 10B characteristic illustrating co-rotatmg vortices 32 These secondary flows result in minimizing or preventing any low pressure hole formation withm fluid flow 10 within duct 25 as illustrated by FIGURE 4C This illustrates how a duct having low-observable characteristics can benefit from the use of very-small-scale effectors to improve fluid flow behavior within duct 25

Flow control system 18 will receive its mput from a sensor system 22 Sensor system 22 may receive mput from conventional flow sensors and the like as known to those skilled m the art or microfabricated electromechanical sensor devices such as those illustrated in FIGURES 5A, 5B and 5C FIGURE 5A illustrates a MEMS sheer sensor 40 This device functions in a manner similar to a hot-film shear stress sensor A small surface flush with the duct wall is mamtamed at a constant temperature The heat flux at the duct wall is then measured This heat flux can be calibrated to shear stress. FIGURE 5B depicts a MEMS pressure sensor 42. FIGURE 5C mdicates a velocity sensor. This device functions m a manner similar to hot-wire anemometers. Electric cuπent is passed through a metal element exposed to the fluid flow The fluid flow convectively cools the element, effectmg a change m its electric resistance. This change in resistance can be related to the velocity magnitude at the sensor through calibration These sensors may be incorporated mto array 13 and communicate to the control system via communication pathways 17 as depicted in FIGURE 6. Additionally, these sensors may be placed elsewhere within fluid flow 10 or may be sensing external flow relative to surface 20. In the case of a ducted fluid flow, sensors may sense the orientation of flow external to duct 25

FIGURE 7 mdicates a duct 50 of a non-conventional shape. This duct may be used in a low-observable aircraft mtake or outlet exhaust, or this unconventional duct shape may be used due to space considerations m additional embodiments. The unconventionally shaped duct 50 may include an aggressive duct offset such as that employed in a low-observable aircraft design The enhancement of fluid flow 10 within this low-observable duct 50 m an aircraft application can determme the size and weight of an all- wmg vehicle and determme the propulsion system requirements and weapons or payload capabilities. Additionally, in an aircraft application, flow control can reduce cyclic fatigue of components located within fluid flow 10 such as an aircraft engme component and the like FIGURES 8 A and 8B illustrate flow separation withm the serpentine duct of FIGURE 7. FIGURE 8A illustrates a secondary flow 10B when the present mvention is not used withm the serpentine duct. FIGURE 8B illustrates the suppression of secondary flow when the present mvention is incorporated onto the ducted surfaces of the serpentine duct Stress peak amplitudes expeπenced by a component withm the fluid flow for a normal ducted flow can be greatly reduced by reducing or eliminating pressure gradients withm the fluid flow 10 Controlled ducted flow reduces these pressure gradients and hence the forcing function amplitudes on a component such as an engme turban blade

Finally, agam in an aircraft application, the low-observable requirements for mlet and exhaust ductmg pose significant challenges The challenges require high aspect ratio and exotic aperture shapmg of ducts or top- mounted mlets for ducts Fluid flow control can be used to mitigate any performance impact on the aircraft Additionally, attack geometries and sensmg internal and external flow conditions at the aircraft and actively manipulating the fluid flow conditions at the aircraft to achieve desired fluid flow conditions at the aircraft will enhance dynamic conditions of the aircraft in flight. Fluid flow may be manipulated to meet several objectives mcludmg: (1) reduced component fatigue, (2) stable fluid flow withm an internal ductmg system, and (3) stable fluid flow external to the aircraft m dynamic geometπes.

In an additional embodiment the present mvention may be used to improve flow behavior m a hydro- dynamic application This may minimize head loss m a piping system, reduce flow noise withm a piping system or over a submerged structure or to control and manipulate hydro-dynamic flow about a watercraft for direction and thrust control

Further embodiments of the present mvention may mclude an handlmg units such as HVAC systems, chemical processors, automobile air mtake manifold or bio-medical applications. However, the present mvention should not be limited to these applications. Rather, the present invention may be applied to any instance of a ducted flow, in particular to a diffused ducted flow

The present mvention provides a system and method for actively manipulating and controlling aerodynamic or hydrodynamic fluid flow over a surface that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods used of fluid flow control

More specifically, the present mvention provides a system and method to control aerodynamic or hydrodynamic fluid flow behavior of a ducted fluid flow using very-small-scale effectors. The system and method for actively manipulating and controlling fluid flow over a surface includes the placement of arrays of very-small-scale effectors on ducted surfaces bounding the ducted fluid flow These very-small-scale effectors are actively manipulated to control the flow behavior of the ducted fluid flow and prevent flow separation within the primary fluid flow The present mvention includes the use of MEMS devices as the very-small-scale effectors. In this case the very-small-scale effectors are smaller than a duct boundary layer thickness of a ducted fluid flow. Furthermore these very-small-scale effectors may be dimensioned at an order of one-tenth of the boundary layer thickness. The present mvention may further sense the flow conditions withm m the pπmary and secondary fluid flow, with a flow sensor system. This relieves computational burdens imposed by prior art systems which failed to sense actual conditions and relied on computationally intensive mathematical models to determme the fluid flow conditions. Further, this elimmates the maccuracies associated with such mathematical models and provides real tune actual feedback. A control system is employed to actively direct the array of very-small-scale effectors m response to the sensed flow conditions to produce a desired fluid flow withm said ducted fluid flow.

Actively controlling ducted fluid flow expands the possible geometries available for ducting systems used to contam such fluid flow. This is highly desirable where exotic geometries are required such as in low- observable tactical aircraft However, the present invention need not be limited to tactical aircraft as low- observable technology has many applications as known to those skilled in the art Furthermore, actively controlling and manipulating a ducted fluid flow can result in reduced fatigue and cyclical stress effects on downstream components such as an aircraft engme or turbine located withm the ducted fluid flow, as the adverse pressure gradients withm the duct can be eliminated.

Operation of components such as an an craft engme or turbine located withm the ducted fluid flow can be enhanced by actively momtormg and controlling the fluid flow behavior to prevent operation failures such as engme stalling mduced by fluid flow

The present mvention provides an important advantage by allowing a dynamic pnmary fluid flow to be actively manipulated by an array of very-small-scale effectors to achieve a greater pressure recovery of the pπmary fluid flow

The present mvention provides another advantage by preventmg flow separation within a ducted or dynamic fluid flow. This is achieved by actively manipulating an array of very-small-scale effectors according to conditions sensed withm the fluid flow to achieve a macroscopic effect. Individually, these very-small-scale effectors mduce vortex generation m the boundary layer. These vortices mduce mixing between the pπmary fluid flow and the secondary fluid flow. The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum m the stream direction are swept along helical paths toward the duct surface to mix with and, to some extent replace low momentum boundary layer flow. By actively controlling the vortex generation across an array of very-small-scale effectors a macro-scale influence can be achieved. This provides a macro-scale source to counter the natural growth of the boundary layer thereby eliminating low energy secondary flow accumulation within the fluid flow. Additionally, since the mduction of these vortices is actively controlled, it is possible for the present invention to respond to changmg conditions effecting the pπmary fluid flow.

Moreover, the present mvention provides yet another advantage by allowing a greater flexibility in the design of a ductmg system to be associated with the primary fluid flow This flexibility allows both mlet and exhaust ductmg to take exotic or serpentine shapes as is often required by other imposed engmeermg design constraints, such as those imposed by low-observable tactical aircraft.

Although the present invention has been described m detail herem with reference to the illustrative embodiments, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is to be further understood, therefore, that numerous changes in the details of the embodiments of this mvention and additional embodiments of this invention will be apparent to, and may be made by, persons of ordinary skill in the art havmg reference to this description. It is contemplated that all such changes and additional embodiments are withm the spirit and true scope of this invention as claimed below.

Claims

WE CLAIM:

1 A method to manipulate flow behavior of a ducted fluid flow using very-small-scale effectors compπsmg the steps of. placmg arrays of very-small-scale effectors on ducted surfaces boundmg the ducted fluid flow; and alteπng a secondary flow structure in a boundary layer of the ducted fluid flow with said arrays of very- small-scale effectors on said ducted surfaces

2. The method of Claim 1, wherem said very-small-scale effectors are smaller than a duct boundary layer thickness of said ducted fluid flow.

3. The method of Claim 1, wherem said very-small-scale effectors are of an order of one-tenth said duct boundary layer thickness of said ducted fluid flow.

4 The method of Claim 1, wherem said very-small-scale effectors are microfabricated elecrro- mechanical systems.

5 The method of Claim 1, wherem said step of alteπng said secondary flow structure compπses actively manipulatmg said arrays of very-small-scale effectors to mduce secondary flows withm the ducted fluid flow via active controls.

6. The method of Claim 5, wherem said secondary flows are co-rotatmg with the ducted fluid flow

7 The method of Claim 5, wherem said secondary flows suppresses flow separation within the ducted fluid flow.

8 The method of Claim 5, further compπsing the steps of sensmg flow conditions withm the ducted fluid flow, with a flow sensor system; and controlling said aπay of very-small-scale effectors m response to said flow conditions, with a control system receivmg an mput from said flow sensor system, to produce a desired fluid flow withm said ducted fluid flow

9. The method of Claim 8, wherem said ducted fluid flow is within a low-observable duct

10. The method of Claim 1, wherem said ducted fluid flow is withm a low-observable duct

11. The method of Claim 8, wherem the ducted fluid flow is manipulated to reduce fatigue effects on downstream components located withm the ducted fluid flow

12. The method of Claim 1, wherem the ducted fluid flow is manipulated to reduce fatigue effects on downstream components located withm the ducted fluid flow.

13. The method of Claim 11, wherem said downstream components comprise an aircraft engine

14. The method of Claim 13, wherem the ducted fluid flow is manipulated to prevent said aircraft engine from stalling.

15. A method to control flow behavior of a ducted fluid flow usmg very-small-scale effectors comprising the steps of: placing arrays of very-small-scale effectors on ducted surfaces boundmg the ducted fluid flow; and altering a secondary flow structure m a boundary layer of the ducted fluid flow with said arrays of very- small-scale effectors on said ducted surfaces to control the flow behavior of the ducted fluid flow and wherein said very-small-scale effectors are smaller than a duct boundary layer thickness of said ducted fluid flow.

16. The method of Claim 15, wherem said very-small-scale effectors are of an order of one-tenth said duct boundary layer thickness of said ducted fluid flow.

17. The method of Claim 15, wherem said very-small-scale effectors are microfabricated electro- mechanical systems.

18. The method of Claim 17, wherem said step of alteπng said second flow structure comprises actively manipulatmg said arrays of very-small-scale effectors induces secondary flows with the ducted fluid flow via active controls.

19. The method of Claim 18, wherem said secondary flows are co-rotating with the ducted fluid flow.

20. The method of Claim 18, wherem said secondary flows suppress flow separation within the ducted fluid flow

21. The method of Claim 18, further compπsmg the steps of: sensing flow conditions withm the ducted fluid flow, with a flow sensor system; and controlling said aπay of very-small-scale effectors in response to said flow conditions, with a control system receiving an input from said flow sensor system, to produce a desired fluid flow withm said ducted fluid flow.

22. The method of Claim 21 , wherem said ducted fluid flow is withm a low-observable duct.

23 The method of Claim 21, wherem the ducted fluid flow is manipulated to reduce fatigue effects on downstream components located within the ducted fluid flow.

24. The method of Claim 23, wherein said downstream components comprise an aircraft engine

25 The method of Claim 23, wherein the ducted fluid flow is manipulated to prevent said aircraft engme from stalling.

26 A system to suppress flow separation of a fluid flow using very-small-scale effectors compπsmg: an arrays of very-small-scale effectors on surfaces bounding the fluid flow and wherem said very-small- scale effectors are microfabπcated electro-mechanical systems that are smaller than a boundary layer thickness of said fluid flow

27. The system of Claim 26 further comprising: a control system coupled to said arrays operable to actively manipulate said aπays on said surfaces to control the flow behavior of the fluid flow.

28 The system of Claim 26, wherem said very-small-scale effectors are of an order of one-tenth said duct boundary layer thickness of said ducted fluid flow.

29 The system of Claim 26, wherem said very-small-scale effectors induce secondary flows within the fluid flow via active control signals from said control system.

30. The system of Claim 26, further comprising: a sensor system operable to sense flow conditions withm the fluid flow and coupled to said control system wherein said control system is operable to manipulate said array of very-small-scale effectors in response to said flow conditions to produce a desired fluid flow withm said fluid flow.

31 The system of Claim 30, wherem the fluid flow is a ducted fluid flow and wherem said surfaces are dueling surfaces

32 The system of Claim 31, wherem said ducting surfaces compπse a low-observable duct.

33 The system of Claim 31 , wherem said ducted fluid flow is actively manipulated by said array of very-small-scale effectors to reduce fatigue effects on downstream components located withm said ducted fluid flow.

34. The system of Claim 33, wherein said downstream components comprise an aircraft engine.

35. The system of Claim 33, wherein the ducted fluid flow is manipulated to improve operational performance of said downstream components.

36. The system of Claim 33, further comprising an external sensor system to sense physical conditions external to said ducted fluid flow which may effect said fluid flow.